Narcolepsy is a sleeping disorder caused by the inability of the brain to control the sleep-wake cycle. The major symptoms of narcolepsy includes, for example, excessive daytime sleepiness, cataplexy induced by emotion (particularly strong joy and surprise), hypnagogic hallucination, and hypnagogic paralysis, and narcolepsy patients are under serious influence in general social life. The prevalence of narcolepsy is assumed to be 0.05-0.2% (0.16-0.18% in Japan), and the prevalence indicates that the disease is not rare.
The therapy of narcolepsy mainly includes a drug therapy and life guidance. For drug therapy, methylphenidate, modafinil and pemoline are used to suppress daytime sleepiness, and tricyclic antidepressant, selective serotonin reuptake inhibitor (SSRI), and serotonin and noradrenaline reuptake inhibitor (SNRI) are used to control cataplexy. While these treatment methods are symptomatic therapy of narcolepsy, they are not basic treatment methods.
In recent years, the relationship between narcolepsy and orexin system dysfunction is attracting attention. Orexins are neuropeptides present in the lateral hypothalamic area, which are two kinds of peptide of orexin-A and orexin-B (hypocretin 1, hypocretin 2 (non-patent document 1)). They bind to orexin 1 receptor (hereinafter to be also referred to as OX1R) and orexin 2 receptor (hereinafter to be also referred to as OX2R), which are G-protein coupled receptors (non-patent document 2). It was suggested from model experiments using mouse and dog that lack of orexin receptor (both OX1R and OX2R are expressed), or lack of OX2R causes narcolepsy (non-patent document 3). Furthermore, it was suggested from model experiments using mouse that the function of OX2R is important for maintaining wakefulness (non-patent document 4, non-patent document 5).
On the other hand, many narcolepsy patients were confirmed to show disappearance of orexin nerves, and decreased orexin concentration (non-patent document 6). Therefore, it is strongly suggested that narcolepsy is highly possibly caused by the lack of orexin.
The orexin receptor is widely expressed in the brain. Orexins are peptides, and are not useful for pharmaceutical use since permeability through the blood-brain barrier is extremely low. Therefore, a low-molecular-weight orexin receptor agonist has been desired. In recent years, a compound with a cyclic guanidine skeleton is reported as a small-molecule OX2R agonist (patent document 1).
In addition, orexin system is considered to not only control the above-mentioned sleep-wake but also appropriately control feeding behavior with emotion and energy balance. A mouse under fasting increases the amount of behavior for searching food by increasing the waking time and decreasing the sleep hours. On the other hand, it was clarified that the waking time and the amount of behavior do not increase in orexin receptor-deficient mouse (non-patent document 7). Moreover, it was suggested that an increase of the leptin sensitivity by OX2R regulates the homeostasis of body weight (non-patent document 8). From these findings, an orexin receptor (particularly OX2R) agonist is a potential therapeutic drug for not only narcolepsy but also diabetes, obesity and metabolic syndrome.
Furthermore, it has been reported that sepsis rats show a decrease in the spontaneous activity and a decrease in the activity of the orexin-containing neurons in the perifornical areas of hypothalamus (non-patent document 9). There is a report that intraventricular administration of orexin to a mouse sepsis model led to an increase in the body temperature and recovery of cardiac function (non-patent document 10). From these, it is possible that an orexin receptor agonist may become a therapeutic drug for sepsis.